Circuits of the Basic TV Set and What They Do - Part 2

In Circuits Of The Basic TV and What They Do - Part 1 we discussed the standby power supply, main power supply, and the horizontal and vertical deflection circuits. In Part 2 of this series we'll dig a bit deeper inside a typical TV set, and cover degaussing circuits, tuner circuits, and common tuner problems.

Degaussing Circuit

Degaussing a picture tube means to demagnetize and remove any stray magnetic fields on the tube. The degaussing circuit is a relatively simple circuit. On older TV sets, the degaussing circuit consisted of a large coil of wire (usually 50 to 100 turns of wire) and a device called a thermistor.

The degaussing coil is located around the large bell around the picture tube. The bell of a picture tube is the surrounding area around the parameter, perpendicular to the face of the picture tube.

Thermistors have a cold resistance rating as well as a hot resistance rating. The cold rating is usually a very low resistance, while the hot is relatively high. When a thermistor is cold and a load such as the degaussing coil is put in series with it, and power is applied, the thermistor starts to heat up. AC power then starts to flow through the coil, thereby createing a magnetic field. The hotter the thermistor gets, the less power flows through the degaussing coil until the thermistor reaches the hot resistive rated level and stops the flow of electricity through the coil, thereby stopping the magnetic field.


There are two types of common thermistors used in TV sets; one has two terminal connections, while the other has three.

The two terminal thermistor will always have a small amount of current flowing through it as long as the AC power is applied. The three terminal type, when cold, will divert the AC power through the terminal connected to the degaussing coil until it heats up to the hot rating. Then, AC power gets sent to the other terminal to power up the rest of the TV set. This type of thermistor can fail and cause the TV set to not start up. The degaussing coil should only measure a few ohms at best, but should not measure high in resistance or shorted.

Testing a Thermistor

The best way to test a thermistor is to make sure the device under test is cold. So, after you unsolder it and take it off the printed circuit board, let it come to room temperature.

With a multimeter in the low resistance mode, the thermistor should check only a few ohms. With the meter leads still attached, take your soldering iron and heat up the thermistor. The resistance should go way up. Now, take away the heat and watch as the resistance starts to fall again until it returns to what you first measured the device at. This thermistor would be considered good.

If the resistance is over 100 ohms cold, I doubt the thermistor is good. If when you put on the heat and the part does not go up in resistance considerably, change the part.

The Relay

On newer types of TV sets, there is still a coil around the parameter of the picture tube bell. However, instead of a thermistor, this circuit uses a small relay to control the voltage flow through the degaussing coil. The small click you hear on picture-tube TV sets when you first turn on the power is this degaussing relay coming on. You should (in a few seconds at most) hear it click off again, and then the picture and sound should follow. This relay is usually controlled by the microprocessor or RC time base circuit.

Testing the Relay Circuit

The best way to test this circuit would be to disconnect the degaussing coil plug from the printed circuit board and measure the resistance between the contact points of the relay when you switch on the TV set. You should also check the relay coil for resistance, which should measure anywhere from 30 to 80 ohms.

Using the Degaussing Circuit

If a strong magnetic field comes close to the front of the TV set, such as child’s toy or a large pair of speakers, and the degaussing circuit is working correctly, the discolorations should go away after a few on/off cycles of the set. You must wait in-between the on and off cycles so the thermistor will cool down. On relay controlled sets, a hard reset will be needed. A hard reset can be done by pulling the main AC plug out of the wall for several minutes.

Some other reasons discoloration can occur on a picture tube face are:

  • The TV set was moved or rotated around
  • A strong lightning strike occured nearby
  • A piece of electronic equipment is placed on or near the TV set

If the auto-degaussing does not seem to work, or does not completely get rid of the discoloration, manually degaussing the tube may be necessary.

Manual Degaussing

Manual degausses are available from most electronic part distributors, and consist of a hundred or so turns of magnet wire in a round coil. They include a line cord and a momentary switch. You simply plug in the coil and flip on the switch.

To manually degauss, bring the coil to within several inches of the screen. Slowly draw the center of the coil toward one edge of the screen and trace the parameter of the screen face. Then, slowly decrease the first parameter unitl the circles get smaller and smaller and you reach the center of the screen. Next, slowly back straight up across the room as you hold the coil. When you are about 5 to 6 feet away, you can release the line switch on the degaussing coil. This must be done in one complete step, and never release the degaussing coil switch until you are away from the TV set.

Never attempt to degauss the inside or back of the set, and near or around the neck of the picture tube. Only degauss the front, sides, and the top of the TV if needed. If you do, you could demagnetize the purity and convergence magnets, which can turn a minor problem into a major and costly repair.

In rare cases, such as when a TV set is dropped or a very large magnet comes close to the front of the TV screen, no amount of degaussing will remove discolorations. This is because the shadow mask inside the picture tube became dislodged or bent. I will discuss the picture tube elements later in this article.

Page 2: Tuner Circuits >>

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Tuner Circuits

There are three basic types of tuner circuits. The first type we will discuss is the rotary tuner, followed by analog varactor tuners, and finally digital tuners.

Rotary Tuners

Rotary tuners are primarily found in older TV sets. These tuners were usually small metal boxes, and each set typcally had two since the VHF and UHF tuning bands were separate.

The VHF rotary tuner had round rotary switches inside, and when you turned the channel by the knob at the front of the TV, a shaft connected to these rotary switches and the knob would simply switch different coils, resistors, and capacitors to the tuner oscillator circuit, thereby changing the frequency to tune different stations.

The UHF rotary tuner was a large variable capacitor that changed the UHF oscillator frequency to receive the different UHF stations. These types of tuners are virtually obsolete these days.

Analog Varactor Tuner

The second type of tuner still used in some of today’s inexpensive or smaller TV sets is the analog varactor tuner. The Varactor tuning system uses what’s called a varactor diode to change the frequency inside the tuner's oscillator and mixer circuits. The varactor diode will change capacitance, depending on how much voltage is across it. The voltage across the varactor diode is varied by a thumbwheel or rotary type potentiometer (a variable resistor). These adjustments vary the voltage going to the tuner. Each individual channel is tuned manually with the different potentiometers and is switched in and out of the tuner circuit by a switch bank or a rotary switch system.

The analog tuners use an input voltage of 0-33 volts DC to control the tuners VCO (Voltage Controlled Oscillator) and there are also 3 pins to select the different bands. The band switching is done by a manual switch, or the microprocessor, to switch between the VHF low band, VHF high band, and UHF band. The disadvantage of the analog tuner is that it is difficult to get stable tuning voltage to the VCO and know what frequency you are receiving.

Digital Tuners

The third and most common type of tuning system in today’s TV sets is the digital tuning system. The digital tuners are controlled by a PLL (Phase-Lock Loop) synthesizer and the microprocessor in the TV's system control circuit.

The digital tuner uses a PLL synthesizer circuit to set the frequency. The synthesizer senses the frequency from the tuner's VCO and compares it with the programmed frequency. The pre-programmed frequencies are in the microprocessor’s firmware. The circuit will then regulate the tuning voltage across the varactor diodes until the frequency from the VCO and the programmed frequencies from the microprocessor are in phase with each other. The bands are also switched by the microprocessor. The digital tuner is very accurate in frequency and is very stable.

The newer HD (High Definition) tuners found in most flat panel or microdisplay TVs are digital tuners that can tune in more then one frequency at a time. These types of tuning systems generally have their own microprocessor control circuits. The typical digital tuner has many surface mounted components on the printed circuit boards. See figures 1 & 2 below.

The terminals of a typical digital tuner are as follows:

  • AGC = Automatic Gain Control. A voltage of 0 to 12V sets the gain of the RF preamplifier.
  • +12V = Power supply to the preamplifier.
  • +33V = Power supply to the PLL tuning system.
  • +5V = Power supply to the PLL synthesizer.
  • SCL = clock to the PLL synthesizer from the microprocessor.
  • SDA = data to the PLL synthesizer from the microprocessor.
  • AS = Address selection for the tuner from the microprocessor. (Band switching)
  • IF = IF out. (IF = Intermediate Frequency)

<< Page 1: Degaussing Circuits | Page 3: Common Tuner Problems >>

More Circuits of the Basic TV Set and What They Do: Part 1 | Part 2 [[page]]

Common Tuner Problems


The most common problems found on the old rotary type tuner were bad connections on the rotary switches inside of the tune, and could usually be cleaned with a tuner contact cleaner. The most common type I used was called “Blue Stuff”. You could spray the cleaner into the tuner through an extension tube attached to the can of tuner spray while you turned the knob in the front. This would fix most problems with these types of tuners.

If these tuners had other problems, the tech would take out the tuner or the tuner assembly, and send it out to a specialty tuner repair shop. These tuner repair places usually charged a flat rate fee to repair tuners, as most still do today.

Analog and Digital

On the modern analog or digital tuners, bad solder connections are the common fault. Sometimes you can get lucky and pinpoint the connections, but most of the time you replace the tuner or have it repaired. For some tuners, it is more feasible to replace the tuner with a new or rebuilt one.

Sometimes the control voltages are the problem with the tuning circuit. The 33 volts is important, so make sure that voltage is not too low and has no AC ripple on the DC line. I have often seen AC ripple on the 33 volt line due to a bad capacitor in the main or sweep power supply. Occasionally, bad microprocessors can cause many tuning problems.

Wrapping Up Part 2

This will conclude Part 2 in this series of articles explaining the basic innards of the typical TV set. In Parts 3 and 4 (coming soon), I will explain the basic system control system, remote control, video, audio, RGB drive, and the picture tube circuits. Stay tuned.

<< Page 2: Tuner Circuits | Comment on this article

More Circuits of the Basic TV Set and What They Do: Part 1 | Part 2


where do i connect to a good ground on a old RCA projection tv

what is it your looking for man???????
Larry Dillon.

thanks for information. I did some circuit theory on electronics some 18 years ago and I found your explanation taking me gently along memory lane. Thanks a lot.
How can I get part3 and 4? and more still keep up to date.

thanks for the info more photos needed

Hay algo que es muy importante y debe reconocerse, el excelente trabajo y dedicacion para todo un publico. Agradezco y sigua adelante que muchos como nosotros, le damos valor a dicho aporte-

Que puede generar en un tv, si este cambia de color la pantalla, morado, rojo etc.
Esto con la intencion de ampliar parte de los expuesto en los temas de la red.

I searched the Internet and I think this article is very well developed. I have a question concerning the tuners, the pin configuration is standard for short. Because I have two TV's first and second samsung emerson two 14-inch, and the two I have problems with the tuners, purchase a new one whose brand is samsung on the label, but the interior is sharp, when placed in the emerson, but placed in short computer, not on, I put the old and the TV lights.

yaa is somethng we ought to rely on since nowadays there are many tv sets its therefore tricky to diagnose them but u are getting to where i need info so where can i find the other info


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